Organic Photosensitive Optoelectronic Devices With Transparent Electrodes

ABSTRACT

A system comprising a plurality of organic photovoltaic cells arranged in a stack disposed between a first electrode and a second electrode, and a resistive load electrically connected across the first electrode and the second electrode. Each cell comprises a rectifying junction at an interface of organic semiconductor materials. There is metal or metal substitute disposed in the stack between each of the cells. At least a first cell and a second cell of the plurality of organic photovoltaic cells have different absorption characteristics. Photocurrent from the plurality of organic photovoltaic cells energizes the resistive load.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of co-pending patent application Ser.No. 10/026,091, filed Dec. 21, 2001, which is a continuation of patentapplication Ser. No. 09/136,342, filed Aug. 19, 1998, now U.S. Pat. No.6,352,777, issued Mar. 5, 2002.

FIELD OF INVENTION

The present invention generally relates to organic thin-filmphotosensitive optoelectronic devices. More specifically, it is directedto organic photosensitive optoelectronic devices, e.g., solar cells andphotodetectors, having transparent electrodes. Further, it is directedto organic photosensitive optoelectronic devices having a transparentcathode which forms a low resistance interface with the adjacent organicsemiconductor layer. Further, it is directed to an organicphotosensitive optoelectronic device consisting of a plurality ofstacked photosensitive optoelectronic subcells, each subcell having oneor more photosensitive optoelectronically active layers and transparentcharge transfer layers.

BACKGROUND OF THE INVENTION

Optoelectronic devices rely on the optical and electronic properties ofmaterials to either produce or detect electromagnetic radiationelectronically or to generate electricity from ambient electromagneticradiation. Photosensitive optoelectronic devices convert electromagneticradiation into electricity. Solar cells, also known as photovoltaic (PV)devices, are specifically used to generate electrical power. PV devicesare used to drive power consuming loads to provide, for example,lighting, heating, or to operate electronic equipment such as computersor remote monitoring or communications equipment. These power generationapplications also often involve the charging of batteries or otherenergy storage devices so that equipment operation may continue whendirect illumination from the sun or other ambient light sources is notavailable. As used herein the term “resistive load” refers to any powerconsuming or storing device, equipment or system. Another type ofphotosensitive optoelectronic device is a photoconductor cell. In thisfunction, signal detection circuitry monitors the resistance of thedevice to detect changes due to the absorption of light. Another type ofphotosensitive optoelectronic device is a photodetector. In operation aphotodetector has a voltage applied and a current detecting circuitmeasures the current generated when the photodetector is exposed toelectromagnetic radiation. A detecting circuit as described herein iscapable of providing a bias voltage to a photodetector and measuring theelectronic response of the photodetector to ambient electromagneticradiation. These three classes of photosensitive optoelectronic devicesmay be characterized according to whether a rectifying junction asdefined below is present and also according to whether the device isoperated with an external applied voltage, also known as a bias or biasvoltage. A photoconductor cell does not have a rectifying junction andis normally operated with a bias. A PV device has at least onerectifying junction and is operated with no bias. A photodetector has atleast one rectifying junction and is usually but not always operatedwith a bias.

Traditionally, photosensitive optoelectronic devices have beenconstructed of a number of inorganic semiconductors, e.g. crystalline,polycrystalline and amorphous silicon, gallium arsenide, cadmiumtelluride and others. Herein the term “semiconductor” denotes materialswhich can conduct electricity when charge carriers are induced bythermal or electromagnetic excitation. The term “photoconductive”generally relates to the process in which electromagnetic radiant energyis absorbed and thereby converted to excitation energy of electriccharge carriers so that the carriers can conduct, i.e., transport,electric charge in a material. The terms “photoconductor” and“photoconductive material” are used herein to refer to semiconductormaterials which are chosen for their property of absorbingelectromagnetic radiation of selected spectral energies to generateelectric charge carriers. Solar cells are characterized by theefficiency with which they can convert incident solar power to usefulelectric power. Devices utilizing crystalline or amorphous silicondominate commercial applications and some have achieved efficiencies of23% or greater. However, efficient crystalline-based devices, especiallyof large surface area, are difficult and expensive to produce due to theproblems inherent in producing large crystals without significantefficiency-degrading defects. On the other hand, high efficiencyamorphous silicon devices still suffer from problems with stability.More recent efforts have focused on the use of organic photovoltaiccells to achieve acceptable photovoltaic conversion efficiencies witheconomical production costs.

PV devices typically have the property that when they are connectedacross a load and are irradiated by light they produce a photogeneratedvoltage. When irradiated without any external electronic load, a PVdevice generates its maximum possible voltage, V open-circuit, orV_(OC). If a PV device is irradiated with its electrical contactsshorted, a maximum short-circuit current, or I_(SC), is produced. Whenactually used to generate power, a PV device is connected to a finiteresistive load and the power output is given by the current voltageproduct, I×V. The maximum total power generated by a PV device isinherently incapable of exceeding the product, I_(SC)×V_(OC). When theload value is optimized for maximum power extraction, the current andvoltage have values, I_(max) and V_(max) respectively. A figure of meritfor solar cells is the fill factor ff defined as: $\begin{matrix}{{ff} = \frac{I_{\max}V_{\max}}{I_{SC}V_{OC}}} & (1)\end{matrix}$where ff is always less than 1 since in actual use I_(SC) and V_(OC) arenever obtained simultaneously. Nonetheless, as ff approaches 1, thedevice is more efficient.

When electromagnetic radiation of an appropriate energy is incident upona semiconductive organic material, for example, an organic molecularcrystal (OMC) material, or a polymer, a photon can be absorbed toproduce an excited molecular state. This is represented symbolically asS₀+hv

S₀*. Here S₀ and S₀* denote ground and excited molecular states,respectively. This energy absorption is associated with the promotion ofan electron from a bound state in the valence band, which may be aπ-bond, to the conduction band, which may be a π*-bond, or equivalently,the promotion of a hole from the conduction band to the valence band. Inorganic thin-film photoconductors, the generated molecular state isgenerally believed to be an exciton, i.e., an electron-hole pair in abound state which is transported as a quasi-particle. The excitons canhave an appreciable life-time before geminate recombination, whichrefers to the process of the original electron and hole recombining witheach other as opposed to recombination with holes or electrons fromother pairs. To produce a photocurrent the electron-hole pair mustbecome separated. If the charges do not separate, they can recombine ina geminate recombination process, either radiatively—re-emitting lightof a lower than incident light energy—, or non-radiatively—with theproduction of heat.

Either of these outcomes is undesirable in a photosensitiveoptoelectronic device. While exciton ionization, or dissociation, is notcompletely understood, it is generally believed to occur in regions ofelectric field occurring at defects, impurities, contacts, interfaces orother inhomogeneities. Frequently, the ionization occurs in the electricfield induced around a crystal defect, denoted, M. This reaction isdenoted S₀*+M

e⁻+h⁺. If the ionization occurs at a random defect in a region ofmaterial without an overall electric field, the generated electron-holepair will likely recombine. To achieve a useful photocurrent, theelectron and hole must be collected separately at respective opposingelectrodes, which are frequently referred to as contacts. This isachieved with the presence of an electric field in the region occupiedby the carriers. In power generation devices, i.e., PV devices, this ispreferably achieved with the use of internally produced electric fieldsthat separate the generated photocarriers. In other photosensitiveoptoelectronic devices, the electric field may be generated by anexternal bias, e.g., in a photoconductor cell, or as a result of thesuperposition of internally and externally generated electric fields,e.g., in a photodetector. In an organic PV device, as in otherphotosensitive optoelectronic devices, it is desirable to separate asmany of the photogenerated electron-hole pairs, or excitons, aspossible. The built-in electric field serves to dissociate the excitonsto produce a photocurrent.

FIG. 1 schematically depicts the photoconductive process in organicsemiconducting materials. Step 101 shows electromagnetic radiationincident upon sample of photoconductive material between two electrodesa and b. In step 102, a photon is absorbed to generate an exciton, i.e.,electron-hole pair, in the bulk. The solid circle schematicallyrepresents an electron while the open circle schematically represents ahole. The curving lines between the hole and electron are an artisticindication that the electron and hole are in an excitonic bound state.In step 103, the exciton diffuses within the bulk photoconductivematerial as indicated by the exciton's closer proximity to electrode a.The exciton may suffer recombination in the bulk material away from anyfield associated with a contact or junction as indicated in step 104. Ifthis occurs the absorbed photon does not contribute to the photocurrent.Preferably the exciton ionizes within the field associated with acontact or junction as indicated by the progression from step 103 tostep 105. However, it is still possible for the newly liberated carriersto recombine as indicated in step 106 before permanently separating andcontributing to the photocurrent. Preferably the carriers separate andrespond to the field near a contact or junction according to the sign oftheir electric charge as indicated by the progression from step 105 tostep 107. That is, in an electric field, indicated by in step 107, holesand electrons move in opposite directions.

To produce internally generated electric fields which occupy asubstantial volume, the usual method is to juxtapose two layers ofmaterial with appropriately selected conductive properties, especiallywith respect to their distribution of molecular quantum energy states.The interface of these two materials is called a photovoltaicheterojunction. In traditional semiconductor theory, materials forforming PV heterojunctions have been denoted as generally being ofeither n, or donor, type or p, or acceptor, type. Here n-type denotesthat the majority carrier type is the electron. This could be viewed asthe material having many electrons in relatively free energy states. Thep-type denotes that the majority carrier type is the hole. Such materialhas many holes in relatively free energy states. The type of thebackground, i.e., not photogenerated, majority carrier concentrationdepends primarily on unintentional doping by defects or impurities. Thetype and concentration of impurities determine the value of the Fermienergy, or level, within the gap between the highest occupied molecularorbital (HOMO) and the lowest unoccupied molecular orbital (LUMO),called the HOMO-LUMO gap. The Fermi energy characterizes the statisticaloccupation of molecular quantum energy states denoted by the value ofenergy for which the probability of occupation is equal to ½. A Fermienergy near the LUMO energy indicates that electrons are the predominantcarrier. A Fermi energy near the HOMO energy indicates that holes arethe predominant carrier. Accordingly, the Fermi energy is a primarycharacterizing property of traditional semiconductors and theprototypical PV heterojunction has traditionally been the p-n interface.

In addition to relative free-carrier concentrations, a significantproperty in organic semiconductors is carrier mobility. Mobilitymeasures the ease with which a charge carrier can move through aconducting material in response to an electric field. As opposed to freecarrier concentrations, carrier mobility is determined in large part byintrinsic properties of the organic material such as crystal symmetryand periodicity. Appropriate symmetry and periodicity can produce higherquantum wavefunction overlap of HOMO levels producing higher holemobility, or similarly, higher overlap of LUMO levels to produce higherelectron mobility. Moreover, the donor or acceptor nature of an organicsemiconductor, e.g., 3,4,9,10-perylenetetracarboxylic dianhydride(PTCDA), may be at odds with the higher carrier mobility. For example,while chemistry arguments suggest a donor, or n-type, character forPTCDA, experiments indicate that hole mobilities exceed electronmobilities by several orders of magnitude so that the hole mobility is acritical factor. The result is that device configuration predictionsfrom donor/acceptor criteria may not be borne out by actual deviceperformance. Thus, in selecting organic materials such as thosedescribed herein for photosensitive optoelectronic devices, it has beenfound that isotype heterojunctions, e.g., p-p, may have rectifyingproperties as good as traditional p-n type heterojunctions, althoughtrue p-n type is generally preferable when possible. Isotypeheterojunctions are discussed further below. Due to these uniqueelectronic properties of organic materials, rather than designating themas “p-type” or “n-type”, the nomenclature of “hole-transporting-layer”(HTL) or “electron-transporting-layer” (ETL) is frequently used. In thisdesignation scheme, an ETL will preferentially be electron conductingand an HTL will preferentially be hole transporting. The term“rectifying” denotes, inter alia, that an interface has an asymmetricconduction characteristic, i.e., the interface supports electroniccharge transport preferably in one direction. Rectification isassociated normally with a built-in electric field which occurs at theheterojunction between appropriately selected materials.

The electrodes, or contacts, used in a photosensitive optoelectronicdevice are an important consideration. In a photosensitiveoptoelectronic device, it is desirable to allow the maximum amount ofambient electromagnetic radiation from the device exterior to beadmitted to the photoconductively active interior region. That is, it isdesirable to get the electromagnetic radiation to where it can beconverted to electricity by photoconductive absorption. This indicatesthat at least one of the electrical contacts should be minimallyabsorbing and minimally reflecting of the incident electromagneticradiation. That is, the contact should be substantially transparent.When used herein, the terms “electrode” and “contact” refer only tolayers that provide a medium for delivering photogenerated power to anexternal circuit or providing a bias voltage to the device. That is, anelectrode, or contact, provides the interface between thephotoconductively active regions of an organic photosensitiveoptoelectronic device and a wire, lead, trace or other means fortransporting the charge carriers to or from the external circuit. Theterm “charge transfer layer” is used herein to refer to layers similarto but different from electrodes in that a charge transfer layer onlydelivers charge carriers from one subsection of an optoelectronic deviceto the adjacent subsection. As used herein, a layer of material or asequence of several layers of different materials is said to be“transparent” when the layer or layers permit at least 50% of theambient electromagnetic radiation in relevant wavelengths to betransmitted through the layer or layers.

When an electrode or charge transfer layer provides the primarymechanism for photovoltaic charge separation, the device is called aSchottky device as discussed further below.

Electrodes or contacts are usually metals or “metal substitutes”. Hereinthe term “metal” is used to embrace both materials composed of anelementally pure metal, e.g., Mg, and also metal alloys which arematerials composed of two or more elementally pure metals, e.g., Mg andAg together, denoted Mg:Ag. Here, the term “metal substitute” refers toa material that is not a metal within the normal definition, but whichhas the metal-like properties that are desired in certain appropriateapplications. Commonly used metal substitutes for electrodes and chargetransfer layers would include wide bandgap semiconductors, for example,transparent conducting oxides such as indium tin oxide (ITO), tin oxide(TO), gallium indium tin oxide (GITO), zinc oxide (ZO) and zinc indiumtin oxide (ZITO). In particular, ITO is a highly doped degenerate n+semiconductor with an optical bandgap of approximately 3.2 eV renderingit transparent to wavelengths greater than approximately 3900 Å. Anothersuitable metal substitute material is the transparent conductive polymerpolyanaline (PANI) and its chemical relatives. Metal substitutes may befurther selected from a wide range of non-metallic materials, whereinthe term “non-metallic” is meant to embrace a wide range of materialsprovided that the material is free of metal in its chemically uncombinedform. When a metal is present in its chemically uncombined form, eitheralone or in combination with one or more other metals as an alloy, themetal may alternatively be referred to as being present in its metallicform or as being a “free metal”. Thus, the metal substitute electrodesof the present invention may sometimes be referred to by one or more ofthe inventors of the present invention as “metal-free” wherein the term“metal-free” is expressly meant to embrace a material free of metal inits chemically uncombined form. Free metals typically have a form ofmetallic bonding that may be thought of as a type of chemical bondingthat results from a sea of valence electrons which are free to move inan electronic conduction band throughout the metal lattice. While metalsubstitutes may contain metal constituents they are “non-metallic” onseveral bases. They are not pure free-metals nor are they alloys offree-metals. Further, these metal substitutes do not have their Fermilevel in a band of conducting states in contrast with true metals. Whenmetals are present in their metallic form, the electronic conductionband tends to provide, among other metallic properties, a highelectrical conductivity as well as a high reflectivity for opticalradiation. Another characteristic of metallic conductors is thetemperature dependence of their conductivity. Metals generally have ahigh conductivity at room temperature which increases as the temperatureis lowered to near absolute zero. Metal substitutes, for example,semiconductors including, inter alia, inorganic, organic, amorphous, orcrystalline, generally have conductivities which decrease as theirtemperature is lowered to near absolute zero.

There are two basic organic photovoltaic device configurations. Thefirst type is the Schottky-type cell with a single species of organicphotoconductive material sandwiched between a pair of metal and/or metalsubstitute contacts. Conventionally, for n-type photoconductors, a highwork function metal, e.g., Au, has been used as the Schottky contact,and for p-type photoconductors, a metal with a low work function, e.g.,Al, Mg, or In has been used as the Schottky contact. The chargeseparation desired in a PV device is induced by exciton dissociation inthe space-charge region associated with the built-in electric field atthe metal/photoconductor interface. Conventionally, such a devicerequires different metal or metal substitute pair combinations ascontacts since use of the same material at both interfaces wouldostensibly produce opposing rectifying junctions. If the same materialis used for both electrodes it has been generally thought that thefields generated at the photoconductor-electrode interfaces arenecessarily equal in magnitude and opposite in direction so that no netphotocurrent is generated in the absence of an external applied voltage.While it is possible for charge separation to occur at both interfacesand be additive, it is generally preferable to have all chargeseparation occurring at one interface. For example, this can be achievedif the non-rectifying interface has little or no barrier to carriertransport, i.e., if it is a relatively low resistance contact. This mayalso be referred to as an “ohmic” contact. In any event, inphotosensitive optoelectronic devices it is generally desirable that theinterfaces either contribute to the net charge separating action orpresent the smallest possible resistance or barrier to carriertransport.

A sample prior art organic Schottky device is shown schematically inFIG. 2A. Contact 2A01 is Ag; organic photoconductive layer 2A02 isPTCDA; and contact 2A03 is ITO. Such a cell was described in an articleby N. Karl, A. Bauer, J. Holzäofel, J. Marktanner, M. Möbus, and F.Stölzle, “Efficient Organic Photovoltaic Cells: The Role of ExcitonicLight Collection, Exciton Diffusion to Interfaces, Internal Fields forCharge Separation, and High Charge Carrier Mobilities”, MolecularCrystals and Liquid Crystals, Vol. 252, pp 243-258, 1994 (hereinafterKarl et al.). Karl et al. noted that while the Ag electrode 2A01 wasphotovoltaically active in such a cell, the ITO electrode very rarelywas photoactive and further that the indications of photovoltaic actionat the ITO electrode had poor statistical certainty. Further, one ofordinary skill in the art would expect contact 2A01 not to betransparent.

The second type of photovoltaic device configuration is the organicbilayer cell. In the bilayer cell, charge separation predominantlyoccurs at the organic heterojunction. The built-in potential isdetermined by the HOMO-LUMO gap energy difference between the twomaterials contacting to form the heterojunction. An isotypeheterojunction has been discussed in an article by S. R. Forrest, L. Y.Leu, F. F. So, and W. Y. Yoon, “Optical and Electrical Properties ofisotype Crystalline Molecular Organic Heterojunctions”, Journal ofApplied Physics, Vol 66, No. 12, 1989 (hereinafter “Forrest, Leu etal.”) and in an article by Forrest, S. R., “Ultrathin Organic FilmsGrown by Organic Molecular Beam Deposition and Related Techniques”,Chemical Reviews, Vol. 97, No. 6, 1997 (hereinafter Forrest, Chem. Rev.1997) both of which are incorporated herein by reference. Forrest, Leuet al. describe two isotype solar cells depicted in FIG. 2B and FIG. 2C.FIG. 2B shows a device consisting of an ITO electrode 2B02 on asubstrate 2B01 covered with a layer 2B03 of copper phthalocyanine (CuPc)and a layer 2B04 of PTCDA with a top electrode 2B05 of In. In a seconddevice, with reference to FIG. 2C, an ITO electrode 2C02 is again placedon a substrate 2C01. Then a CuPc layer 2C03 and a3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI) layer 2C04are placed in order with a Ag electrode 2C05 on top. This prior art hadonly one transparent electrode and it was on the bottom of the device.It was also noted in this reference that these organic photovoltaicdevices suffered from a high series resistance.

As in the case of Schottky devices, unless an interface, at a contact,for example, is contributing to the charge separation, it is desirablethat the interface produce the smallest possible obstruction to freecurrent flow. In bilayer devices, since the dominant charge separatingregion is located near the heterojunction, it is desirable for theinterfaces at the electrodes to have the smallest possible resistance.In particular, it is known in the art to use thin metal layers as lowresistance, or ohmic, electrodes, or contacts. When ohmic contacts aredesired, a high work function metal, e.g., Au, has been used as thepositive electrode, i.e., anode, in photosensitive optoelectronicdevices. Similarly, a low work function metal, e.g., Al, Mg, or In, hasbeen used to make an ohmic negative electrode, i.e., cathode.

Herein, the term “cathode” is used in the following manner. In a PVdevice under ambient irradiation and connected with a resistive load andwith no externally applied voltage, e.g., a solar cell, electrons moveto the cathode from the adjacent photoconducting material. With anapplied bias voltage, electrons may move from the cathode to theadjacent photoconducting material, or vice versa, depending on thedirection and magnitude of the applied voltage. For example, under“forward-bias” a negative bias is applied to the cathode. If themagnitude of the forward-bias equals that of the internally generatedpotential there will be no net current through the device. If theforward-bias potential exceeds the internal potential in magnitude therewill be a current in the opposite direction from the non-biasedsituation. In this later forward-bias situation, electrons move from thecathode into the adjacent photoconductive organic layer. Under“reverse-bias”, a positive bias is applied to the cathode and anyelectrons which can move do so in the same direction as in the no biassituation. A reverse-biased device generally has little or no currentflow until it is irradiated. Similarly, the term “anode” is used hereinsuch that in a solar cell under illumination, holes move to the anodefrom the adjacent photoconducting material, which is equivalent toelectrons moving in the opposite manner. The application of an externalvoltage to the device structure will alter the flow of the carriers atthe anode/photoconductor interface in a complementary fashion to thatdescribed for the cathode and in a manner understood by those ofordinary skill in the art. It will be noted that as the terms are usedherein anodes and cathodes may be electrodes or charge transfer layers.

Further, as discussed above, in non-Schottky photosensitiveoptoelectronic devices it is similarly desirable for the electrodes notmerely to form ohmic contacts but also to have high opticaltransparency. Transparency requires both low reflectivity and lowabsorption. Metals have the desired low resistance contact properties;however, they can produce significant conversion efficiency reductionsdue to reflection of ambient radiation away from the device. Also, metalelectrodes can absorb significant amounts of electromagnetic radiation,especially in thick layers. Therefore, it has been desirable to find lowresistance, high transparency electrode materials and structures. Inparticular, the metal substitute ITO has the desired optical properties.It is also known in the art that ITO functions well as an anode inorganic optoelectronic devices. However, it had not been previouslythought that ITO or other metal substitutes could make low resistancecathodes for organic optoelectronic devices. Solar cells had beendisclosed in which a highly transparent ITO layer may have functioned asa cathode in some cases, but such ITO cathodes were only disclosed ashaving been prepared by depositing the charge-carrying organic layeronto the ITO layer by Karl et al. and Whitlock, J. B., Panayotatos, P.,Sharma, G. D., Cox, M. D., Savers, R. R., and Bird, G. R.,“Investigations of Materials and Device Structures for OrganicSemiconductor Solar Cells,” Optical Eng., Vol. 32, No. 8, 1921-1934(August 1993), (Whitlock et al.).

Prior art PV devices, e.g., FIGS. 2A and 2B, have only utilizednon-metallic materials, e.g., indium tin oxide (ITO), as one electrodeof the photovoltaic device. The other electrode has traditionally been anon-transparent metallic layer, e.g., aluminum, indium, gold, tin,silver, magnesium, lithium, etc. or their alloys, selected on the basisof work function as discussed above. U.S. Pat. No. 5,703,436 to Forrest,S. R. et al. (hereinafter Forrest '436), incorporated herein byreference, describes a technique for fabricating organic photoemissivedevices (TOLEDs: Transparent Organic Light Emitting Diodes) having atransparent cathode deposited onto an organic ETL by depositing a thinmetallic layer, e.g., Mg:Ag, onto the organic ETL and then sputterdepositing an ITO layer onto the Mg:Ag layer. Such a cathode having theITO layer sputter deposited onto a Mg:Ag layer is referred to herein asa “composite ITO/Mg:Ag cathode”. The composite ITO/Mg:Ag cathode hashigh transmission as well as low resistance properties.

It is known in the art of inorganic solar cells to stack multiplephotovoltaic cells to create an inorganic multisection solar cell withtransparent metallic layers. For example, U.S. Pat. No. 4,255,211 toFrass (hereinafter “Frass '211”) discloses a stacked cell arrangement.However, the photolithographic techniques used to fabricate inorganicelectronic devices are typically inapplicable to production of organicoptoelectronic devices. Photolithography generally involves depositionof metallic layers and inorganic semiconductive layers followed byadditional steps of masking and etching. The etching steps involve useof strong solvents which can dissolve the relatively fragile organicsemiconductor materials that are suitable for organic photovoltaicdevices. Therefore, organic photosensitive optoelectronic devicefabrication techniques typically avoid this type of liquid etchingprocess in which deposited material is removed after an organic layerhas been deposited. Instead, device layers are generally depositedsequentially with techniques such as evaporation or sputtering. Accessto electrodes is generally implemented using masking or dry etchingduring deposition. This constraint presents a challenge to fabricationof a stacked organic optoelectronic device for which electrode access tothe intervening layers in the stack is desired. Thus, it is believedthat all prior art stacked cells have the individual photovoltaic cellselectrically connected internally and only in series.

For inorganic photovoltaic devices, series connection is notparticularly disadvantageous. However, due to the high series resistanceof the organic photovoltaic devices noted above, a series configurationis undesirable for power applications due to the reduced efficiency.Forrest, Chem. Rev. 1997 reported that high series resistance in organicsolar cells leads to space-charge build-up as power levels are raisedwith increasing incident light intensity. This leads to degradation ofthe photocurrent, I_(max), effectively reducing the fill factor andtherefore the efficiency. Moreover, what is believed to be the onlypreviously disclosed organic solar cell with more than one photovoltaicsubcell was a tandem, i.e., two PV subcells, with the subcells connectedin series. See Effect of Thin Gold Interstitial-layer on thePhotovoltaic Properties of Tandem Organic Solar Cell, Hiramoto, M.;Suezaki, M.; Yokoyama, M; Chemistry Letters 1990, 327 (hereinafter“Hiramoto”). Referring to FIG. 2D, substrate 2D01 is glass; 2D02 is ITO;2D03 is Me-PTC (700 Å); 2D04 is H₂Pc (500 Å); 2D05 is Au (<30 Å); 2D06is Me-PTC (700 Å); and 2D07 is H₂Pc (500 Å). In addition, there is an Au(200 Å) electrode (not shown) on top of the layer 2D07. This device hasthe subcells electrically connected internally and in series, thusavoiding the problem of devising a means to make external contact to anelectrode within the middle of a stack of organic semiconductingmaterial. Hiramoto's organic tandem devices have just two electrodes:one on top and bottom used to make external connections plusrecombination layer 2D05 which electrically “floats” between the twosubcells. Only one of the electrodes, bottom ITO layer 2D02 wastransparent. Top Au layer (not shown) was 200 Å thick and thereforenon-transparent. Further, for the reasons noted above, series connectionis not an optimal configuration in stacked organic photovoltaic devicesfor high power applications.

A solar cell may be viewed as a photodiode with no applied bias. Theinternal electric field generates a photocurrent when light is incidenton the solar cell and the current drives a resistive load for theextraction of power. On the other hand, a photodetector may be viewed asa diode with no externally applied bias voltage or a finite externallyapplied bias voltage. When electromagnetic radiation is incident upon aphotodetector with a bias, the current increases from its dark value toa value proportional to the number of photogenerated carriers and theincrease may be measured with external circuitry. If a photodiode isoperated with no applied bias, an external circuit may be used tomeasure the photogenerated voltage and achieve photodetection. While thesame general configuration of electrodes, charge transfer layers andphotoconductive layers may be used alternatively as a solar cell or as aphotodetector, a configuration optimized for one purpose is generallynot optimal for another. For example, photosensitive optoelectronicdevices produced as solar cells are designed to convert as much of theavailable solar spectrum as possible to electricity. Therefore, a broadspectral response over the entire visible spectrum is desirable. On theother hand, a photodetector may be desired which has a photosensitiveresponse over a narrow spectral range or over a range outside thevisible spectrum.

Organic PV devices typically have relatively low quantum yield (theratio of photons absorbed to carrier pairs generated, or electromagneticradiation to electricity conversion efficiency), being on the order of1% or less. This is in part thought to be due to the second order natureof the intrinsic photoconductive process, that is, carrier generationrequires exciton generation, diffusion and ionization, as describedabove. In order to increase these yields, materials and deviceconfigurations are desirable which can enhance the quantum yield and,therefore, the power conversion efficiency.

Forrest Chem. Rev. 1997 and Arbour, C., Armstrong, N. R., Brina, R.,Collins, G., Danziger, J.-P., Lee, P., Nebesny, K. W., Pankow, J.,Waite, S., “Surface Chemistries and Photoelectrochemistries of Thin FilmMolecular Semiconductor Materials”, Molecular Crystals and LiquidCrystals, 1990, 183, 307, (hereinafter Arbour et al.), incorporatedherein by reference in its entirety, disclose that alternating thinmultilayer stacks of similar type photoconductors could be used toenhance photogenerated carrier collection efficiency over that using asingle layer structure. Further, these sources describe multiple quantumwell (MQW) structures in which quantum size effects occur when the layerthicknesses become comparable to the exciton dimensions.

SUMMARY AND OBJECTS OF INVENTION

The present invention is directed to organic photosensitiveoptoelectronic devices utilizing transparent electrodes, in particular,devices that include an organic photosensitive optoelectronic cellcomprised of at least one pair of two transparent electrodes, that is, atransparent cathode and a transparent anode, or devices that have atransparent electrode in superposed relationship upon the top surface ofa substrate with at least one photoconductive organic layer disposedbetween the electrode and the substrate. More specifically, the organicphotosensitive optoelectronic devices of the present invention may becomprised of a transparent cathode that is highly transparent and/orhighly efficient. As representative embodiments, such transparentcathodes may be the highly transparent, highly efficient and/or lowresistance non-metallic or metallic/non-metallic composite cathodes,such as disclosed in the co-pending application Ser. Nos. 08/964,863(now U.S. Pat. No. 6,469,437; hereinafter “Parthasarathy '437”) and09/054,707 (now U.S. Pat. No. 6,420,031; hereinafter “Parthasarathy'031”) or in Forrest '436, each of which being incorporated in itsentirety by reference.

The organic photosensitive optoelectronic devices of the presentinvention may function as a solar cell, photodetector or photocell.Whenever the organic photosensitive optoelectronic devices of thepresent invention function as solar cells, the materials used in thephotoconductive organic layer or layers and the thicknesses thereof maybe selected, for example, to optimize the external quantum efficiency ofthe device. Whenever the organic photosensitive optoelectronic devicesof the present invention function as photodetectors or photocells, thematerials used in the photoconductive organic layer or layers and thethicknesses thereof may be selected, for example, to maximize thesensitivity of the device to desired spectral regions. In each case, useof transparent electrodes, or even only a transparent top electrode,makes it possible for substantially higher external quantum efficienciesand/or photosensitivities in selected spectral regions to be realizedcompared to when one or more of the electrodes can cause substantialtransmission losses due to absorption and/or reflection losses.

In addition to the organic photosensitive optoelectronic devices thatmay be comprised of two transparent electrodes or a transparent topelectrode, the present invention is further directed to organicphotosensitive optoelectronic devices having the unique geometric andelectrical configurations that may be fabricated using stacked cellswith transparent electrodes. In particular, the organic photosensitiveoptoelectronic device may be a stacked device comprised of a pluralityof subcells in superposed relation to each other on the surface of asubstrate. The materials and thicknesses of the individual subcells maybe selected, for example, together with selecting the total number ofsubcells that are included in the stacked photosensitive optoelectronicdevice, so as to optimize the external quantum efficiency of thephotosensitive optoelectronic device.

In particular, for stacked photosensitive optoelectronic devicesconfigured to be electrically connected in parallel, the thicknesses ofthe individual subcells may be adjusted so that in combination withselecting the total number of subcells in the stacked device, theexternal quantum efficiency of the device may be optimized so as toobtain an external quantum efficiency that is higher than that which ispossible for a single cell. The term “external quantum efficiency” isused herein to refer to the efficiency with which a photosensitiveoptoelectronic device is capable of converting the total incidentradiation into electrical power, as distinct from the term “internalquantum efficiency,” which is used herein to refer to the efficiencywith which a photosensitive optoelectronic device is capable ofconverting the absorbed radiation into electrical power. Using theseterms, a stacked photosensitive optoelectronic device with anelectrically parallel configuration may be designed to achieve anexternal quantum efficiency, under a given set of ambient radiationconditions, that approaches the maximum internal quantum efficiency thatmay be achieved for an individual subcell under such ambient conditions.

This result may be achieved by considering several guidelines that maybe used in the selection of layer thicknesses. It is desirable for theexciton diffusion length, L_(D), to be greater than or comparable to thelayer thickness, L, since it is believed that most exciton dissociationwill occur at an interface. If L_(D) is less than L, then many excitonsmay recombine before dissociation. It is further desirable for the totalphotoconductive layer thickness to be of the order of theelectromagnetic radiation absorption length, 1/α (where α is theabsorption coefficient), so that nearly all of the radiation incident onthe solar cell is absorbed to produce excitons. However, the layerthicknesses should not be so large compared to the extent of theheterojunction electric fields that many excitons get generated in afield-free region. One reason for this is that the fields help todissociate the excitons. Another reason is that if an excitondissociates in a field-free region, it is more likely to suffer geminantrecombination and contribute nothing to the photocurrent. Furthermore,the photoconductive layer thickness should be as thin as possible toavoid excess series resistance due to the high bulk resistivity oforganic semiconductors.

Accordingly, these competing guidelines inherently require tradeoffs tobe made in selecting the thickness of the photoconductive organic layersof a photosensitive optoelectronic cell. Thus, on the one hand, athickness that is comparable or larger than the absorption length isdesirable in order to absorb the maximum amount of incident radiation.On the other hand, as the photoconductive layer thickness increases, twoundesirable effects are increased. One is that due to the high seriesresistance of organic semiconductors, an increased organic layerthickness increases device resistance and reduces efficiency. Anotherundesirable effect is that increasing the photoconductive layerthickness increases the likelihood that excitons will be generated farfrom the effective field at a charge-separating interface, resulting inenhanced probability of geminate recombination and, again, reducedefficiency. Therefore, a device configuration is desirable whichbalances between these competing effects in a manner that produces ahigh quantum efficiency for the overall device.

In particular, by taking the above-noted competing effects into account,that is, the absorption length of the photoconductive materials in thedevice, the diffusion length of the excitons in these materials, thephotocurrent generation efficiency of these excitons, and theresistivity of these materials, the thickness of the layers in anindividual cell may be adjusted so as to obtain a maximum internalquantum efficiency for those particular materials for a given set ofambient radiation conditions. Since the diffusion length of the excitonstends to have a relatively small value and the resistivity of typicalphotoconductive materials tends to be relatively large, an optimalsubcell with respect to achieving the maximum internal quantumefficiency would typically be a relatively thin device. However, sincethe absorption length for such photoconductive organic materials tendsto be relatively large as compared with the exciton diffusion length,such thin optimal photosensitive optoelectronic subcells, which may havethe maximum internal quantum efficiency, would tend to have a relativelylow external quantum efficiency, since only a small fraction of theincident radiation would be absorbed by such optimal subcells.

So as to improve the external quantum efficiency of an individualsubcell, the thickness of the photoconductive organic layers may beincreased so as to absorb significantly more incident radiation.Although the internal quantum efficiency for converting the additionallyabsorbed radiation into electrical power might gradually decrease as thethickness is increased beyond its optimal subcell thickness, theexternal quantum efficiency of the subcell would still increase until acertain thickness is reached where no further increase in absorptioncould produce an increase in external quantum efficiency. Since theinternal quantum efficiency of the subcell tends to drop rather sharplyas the thickness of the photoconductive layers increases much beyond thediffusion length of the photogenerated excitons, the maximum externalquantum efficiency of the subcell may be achieved well before thethickness of the thicker subcell is sufficient to absorb substantiallyall the incident radiation. Thus, the maximum external quantumefficiency that may be achieved using this single, thicker-cell approachis limited not only by the fact that the subcell thickness may besignificantly greater than that desired for achieving the maximuminternal quantum efficiency but, in addition, such thicker subcells maystill not absorb all the incident radiation. Thus, due to both of theseeffects, the maximum external quantum efficiency of the thicker subcellwould be expected to be significantly less than the maximum internalquantum efficiency that can be achieved for an optimal subcell havingthe optimal thickness.

A particular feature of the present invention having the stacked organicphotosensitive optoelectronic device with the electrically parallelconfiguration is that instead of attempting to improve the externalquantum efficiency by increasing the thickness of a single subcell,which sacrifices the internal quantum efficiency, subcells that have athickness that is optimal or near optimal for achieving the maximuminternal quantum efficiency may be used to fabricate a stackedstructure. The total number of such optimal subcells that are includedin the stacked structure may be increased so as to provide an increasein absorption of the incident radiation with the total number beinglimited by that which produces no further increase in the externalquantum efficiency. The net result of this approach for improving theexternal quantum efficiency is that a stacked organic photosensitiveoptoelectronic device can be made to have an external quantum efficiencyapproaching the maximum value of the internal quantum efficiency thatcan be achieved for an individual optimal subcell. The improved externalquantum efficiency of the stacked devices may be attributed in largepart to the fact that the subcells of the stacked device may becomprised of pairs of transparent electrodes and, in some cases, also ofa transparent top electrode.

Taking into account that the additional subcells of the stacked devicetend to introduce additional losses, such as that due to the residualreflectivity of the transparent electrodes, the maximum external quantumefficiency that can be achieved for a fully optimized stacked devicewould typically be somewhat less than the internal quantum efficiency ofan optimal subcell. Nevertheless, using the methods of the presentinvention for optimizing the external quantum efficiency of an organicphotosensitive optoelectronic device, substantially higher externalquantum efficiencies may be achieved for a stacked device than arepossible for a device having a single cell, which is optimized forexternal quantum efficiency at the expense of internal quantumefficiency.

Since the organic photosensitive optoelectronic devices of the presentinvention may be desired for widely varying ambient radiationconditions, for example, with respect to the intensity of incidentradiation and/or with respect to the spectral distribution of theincident radiation, the photoconductive organic materials, and the layerthicknesses thereof, may be selected so as to be optimized for a givenset of ambient conditions. For example, the photoconductive organicmaterials may be selected to have absorption maxima in selected spectralregions. Since the photoconductive organic materials that may be used ina photosensitive optoelectronic cell may typically have absorptionmaxima only over a limited spectral range, it is an additional featureof the present invention that the stacked photosensitive optoelectronicdevices may be comprised of different types of cells havingphotoconductive organic materials with different absorptioncharacteristics so as to more effectively utilize the entire spectralrange of the incident radiation.

When the term “subcell” is used hereafter, it may refer to an organicphotosensitive optoelectronic construction of the unilayer, bilayer ormultilayer type. When a subcell is used individually as a photosensitiveoptoelectronic device, it typically includes a complete set ofelectrodes, i.e., positive and negative. As disclosed herein, in somestacked configurations it is possible for adjacent subcells to utilizecommon, i.e., shared, electrode or charge transfer layers. In othercases, adjacent subcells do not share common electrodes or chargetransfer layers. The term “subcell” is disclosed herein to encompass thesubunit construction regardless of whether each subunit has its owndistinct electrodes or shares electrodes or charge transfer layers withadjacent subunits. Herein the terms “cell”, “subcell”, “unit”,“subunit”, “section”, and “subsection” are used interchangeably to referto a photoconductive layer or set of layers and the adjoining electrodesor charge transfer layers. As used herein, the terms “stack”, “stacked”,“multisection” and “multicell” refer to any optoelectronic device withmultiple layers of a photoconductive material separated by one or moreelectrode or charge transfer layers.

Since the stacked subcells of the solar cell may be fabricated usingvacuum deposition techniques that allow external electrical connectionsto be made to the electrodes separating the subcells, each of thesubcells in the device may be electrically connected either in parallelor in series, depending on whether the power and/or voltage generated bythe solar cell is to be maximized. The improved external quantumefficiency that may be achieved for the stacked solar cells of thepresent invention may also be attributed to the fact that the subcellsof the stacked solar cell may be electrically connected in parallelsince a parallel electrical configuration permits substantially higherfill factors to be realized than when the subcells are connected inseries. It is believed that this parallel electrical configuration ofthe stacked subcells is a further unique aspect of the presentinvention.

Although the high series resistance of photoconductive organic materialsinhibits use of subcells in a series configuration for high powerapplications, there are certain applications, for example, in operatingliquid crystal displays (LCD), for which a higher voltage may berequired, but only at low current and, thus, at low power levels. Forthis type of application, stacked, series-connected solar cells may besuitable for providing the required voltage to the LCD. In the case whenthe solar cell is comprised of subcells electrically connected in seriesso as to produce such a higher voltage device, the stacked solar cellmay be fabricated so as to have each subcell producing approximately thesame current so to reduce inefficiency. For example, if the incidentradiation passes through in only one direction, the stacked subcells mayhave an increasing thickness with the outermost subcell, which is mostdirectly exposed to the incident radiation, being the thinnest.Alternatively, if the subcells are superposed on a reflective surface,the thicknesses of the individual subcells may be adjusted to accountfor the total combined radiation admitted to each subcell from theoriginal and reflected directions.

Further, it may be desirable to have a direct current power supplycapable of producing a number of different voltages. For thisapplication, external connections to intervening electrodes could havegreat utility and are not believed to have been previously disclosed.Accordingly, in addition to being capable of providing the maximumvoltage that is generated across the entire set of subcells, the stackedsolar cells of the present invention may also be used to providemultiple voltages from a single power source by tapping a selectedvoltage from a selected subset of subcells.

The present invention may be further described as being directed towarda method of fabricating photosensitive optoelectronic devices comprisingfabricating a first photosensitive optoelectronic subcell on a substrateso as to form a photosensitive optoelectronic cell capable of producinga given external quantum efficiency, and fabricating a secondphotosensitive optoelectronic subcell in superposed relationship uponthe top surface of the first photosensitive optoelectronic subcell so asto form a stacked photosensitive optoelectronic device so as to increasethe external quantum efficiency capability of the photosensitiveoptoelectronic cell, wherein at least one of the subcells of the stackedphotosensitive optoelectronic cell is comprised of a pair of transparentelectrodes.

The present invention may be further described as being directed towarda method of fabricating a series stacked organic photosensitiveoptoelectronic device comprising fabricating a first organicphotosensitive optoelectronic subcell on a substrate so as to form anorganic photosensitive optoelectronic device capable of producing agiven voltage, and fabricating a second organic photosensitiveoptoelectronic subcell in superposed relationship upon the top surfaceof the first organic photosensitive optoelectronic subcell so as to forma stacked organic photosensitive optoelectronic device and so as toincrease the voltage capability of the organic photosensitiveoptoelectronic device, wherein the subcells of the stacked organicphotosensitive optoelectronic cell are comprised of a pair oftransparent electrode layers and the first subcell and the secondsubcell are electrically connected in series.

The present invention may be further described as being directed towarda method of fabricating a parallel stacked organic photosensitiveoptoelectronic device comprising fabricating a first organicphotosensitive optoelectronic subcell on a substrate so as to form anorganic photosensitive optoelectronic device capable of producing agiven external quantum efficiency, and fabricating a second organicphotosensitive optoelectronic subcell in superposed relationship uponthe top surface of the first organic photosensitive optoelectronicsubcell so as to form a stacked organic photosensitive optoelectronicdevice so that the external quantum efficiency capability of the organicphotosensitive optoelectronic device is increased, wherein the firstsubcell and the second subcell are electrically connected in parallel.

The present invention may be further described as being directed towarda mixed electrical configuration stacked organic photosensitiveoptoelectronic device comprising a substrate having a proximal surfaceand a distal surface, and a plurality of subassemblies of organicphotosensitive optoelectronic subcells, each of the subcells having acathode and an anode, each of the cathode and anode being an electrodelayer or a charge transfer layer, the subcells in superposed relationwith each other and with the distal surface of the substrate, each ofthe subassemblies of subcells comprising a plurality of subcellselectrically connected in parallel or a plurality of subcellselectrically connected in series, wherein the subassemblies areelectrically connected to each other in series or in parallel such thatthe device includes subcells electrically arranged in series andparallel, so that the device is capable of producing a voltage higherthan possible with a completely parallel arrangement with the samematerials and with higher external quantum efficiency than a completelyseries arrangement for producing the same voltage.

The present invention may be further described as being directed towarda method of fabricating a mixed electrical configuration stacked organicphotosensitive optoelectronic device comprising: fabricating a firstorganic photosensitive optoelectronic subcell on a substrate so as toform an organic photosensitive optoelectronic device; fabricating asecond organic photosensitive optoelectronic subcell in superposedrelationship upon the top surface of the first organic photosensitiveoptoelectronic subcell so as to form a first stacked organicphotosensitive optoelectronic subassembly comprised of the first subcelland the second subcell electrically connected in series; fabricating athird organic photosensitive optoelectronic subcell in superposedrelationship upon the top surface of the second organic photosensitiveoptoelectronic subcell; and fabricating a fourth organic photosensitiveoptoelectronic subcell in superposed relationship upon the top surfaceof the third organic photosensitive optoelectronic subcell so as to forma second stacked organic photosensitive optoelectronic subassemblycomprising the third subcell and the fourth subcell electricallyconnected in series, wherein the first stacked organic photosensitiveoptoelectronic subassembly and the second stacked organic photosensitiveoptoelectronic subassembly are electrically connected in parallel.

Representative embodiments may also comprise transparent charge transferlayers. As described herein charge transfer layers are distinguishedfrom ETL and HTL layers by the fact that charge transfer layers arefrequently, but not necessarily, inorganic and they are generally chosennot to be photoconductively active. That is, the electrodes and chargetransfer layers preferably do not absorb electromagnetic radiation forconversion to electrical or thermal forms of energy. Therefore,transparent low reflectivity electrodes and charge transfer layers aregenerally preferred in the present invention. In addition, the electrodeand charge transfer layer electronic properties are important. Incertain device configurations one or more of the electrodes or chargetransfer layers may be electronically active. For example, as discussedabove, an electrode or charge transfer layer may provide an interfacialregion for dissociating or recombining excitons, or it may provide arectifying interface. In other device configurations, it is desired thatthe electrode or charge transfer layer have minimal electronic activityand instead serve primarily as a low resistance means for delivering thephotogenerated current to the external circuitry or to the adjacentsubsection of a multisection device. Moreover, in PV devices, highcontact or charge transfer layer resistance is detrimental in manyapplications since the resulting increased series resistance limitspower output.

The preferred embodiments of the present invention include, as one ormore of the transparent electrodes of the optoelectronic device, ahighly transparent, non-metallic, low resistance cathode such asdisclosed in Parthasarathy '031, or a highly efficient, low resistancemetallic/non-metallic composite cathode such as disclosed in Forrest'436. Each type of cathode is preferably prepared in a fabricationprocess that includes the step of sputter depositing an ITO layer ontoeither an organic material, such as copper phthalocyanine (CuPc), PTCDAand PTCBI, to form a highly transparent, non-metallic, low resistancecathode or onto a thin Mg:Ag layer to form a highly efficient, lowresistance metallic/non-metallic composite cathode. Parthasarathy '031discloses that an ITO layer onto which an organic layer had beendeposited, instead of an organic layer onto which the ITO layer had beendeposited, does not function as an efficient cathode.

In summary, it is an object of the present invention to provide anorganic photosensitive optoelectronic device with two transparentelectrodes.

More specifically, it is an object of the present invention to provide astacked solar cell comprised of one or more subcells comprised of twotransparent electrodes.

It is another object of the present invention to provide a stacked solarcell capable of operating with a high external quantum efficiency.

It is a still more specific object of the present invention to provide astacked solar cell capable of operating with an external quantumefficiency that approaches the maximum internal quantum efficiency of anoptimal PV subcell.

It is yet another object of the present invention to provide a stackedsolar cell capable of operating with a higher voltage than can beprovided by a single subcell.

Another object of the present invention is to provide an organicphotosensitive optoelectronic device including multiple quantum wellstructures.

A further object of the present invention is to provide a stackedorganic photosensitive optoelectronic device comprised of multipleorganic photosensitive optoelectronic subcells with the subcells havingexternal electrical connections.

Another object of the present invention is to provide an organicphotosensitive optoelectronic device with improved absorption ofincident radiation for more efficient photogeneration of chargecarriers.

It is a further objective of the present invention to provide an organicphotosensitive optoelectronic device with an improved V_(OC) and animproved I_(SC).

Another object of the present invention is to provide a stacked organicphotosensitive optoelectronic device having parallel electricalinterconnection of the subcells.

A further object of the present invention is to provide a stackedorganic photosensitive optoelectronic device comprised of multipleorganic photovoltaic subcells with transparent electrodes and having asubstantially reflective bottom layer to increase overallelectromagnetic radiation absorption by capturing the electromagneticradiation reflected by the bottom layer.

Yet another object of the present invention is to provide organicphotosensitive optoelectronic devices including a conductive or aninsulating substrate.

A further object of the present invention is to provide organicphotosensitive optoelectronic devices including a rigid or a flexiblesubstrate.

A further object of the present invention is to provide organicphotosensitive optoelectronic wherein the organic materials used arepolymeric or non-polymeric thin films.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present invention will be morereadily apparent from the following detailed description of exemplaryembodiments taken in conjunction with the attached drawings. It will benoted that for convenience all illustrations of devices show the heightdimension exaggerated in relation to the width.

FIG. 1 illustrates a simple prior art model of photoconductivity inorganic materials.

FIGS. 2A, 2B, 2C and 2D schematically depict prior art organicphotovoltaic devices.

FIG. 3 schematically depicts an organic photosensitive optoelectronicdevice with transparent non-metallic electrodes and a single organiclayer.

FIGS. 4A and 4B schematically depict organic photosensitiveoptoelectronic device with transparent electrodes and two organiclayers.

FIG. 5 illustrates the energy levels of an isotype bilayer interface.

FIG. 6 schematically depicts an organic photosensitive optoelectronicdevice with transparent non-metallic electrodes and more than twoorganic layers.

FIG. 7 schematically depicts a stacked organic photosensitiveoptoelectronic device arrangement where each subunit has one organiclayer.

FIGS. 8A, 8B, 8C and 8D schematically depict stacked organicphotosensitive optoelectronic device arrangements where each subunit hastwo organic layers.

FIG. 9 schematically depicts a stacked organic photosensitiveoptoelectronic device arrangement where each unit has more than twoorganic layers.

FIGS. 10A and 10B schematically depict organic photosensitiveoptoelectronic devices employing multiple stacked photoconductivelayers.

FIGS. 11A and 11B schematically depict organic photosensitive devicesutilizing reflective layers.

FIG. 12 is a stacked photosensitive optoelectronic device with parallelinterconnection of subunits after fabrication in a more realisticdepiction.

DETAILED DESCRIPTION

In an exemplary embodiment of the present invention, as shown in FIG. 3,the device structure 300 is deposited onto an insulating substrate 301.First electrode 302 is transparent and comprises, e.g., ITO deposited byconventional techniques or available predeposited on commercialsubstrates to an approximate thickness of 1000-4000 Å, preferably lessthan 2000 Å and most preferably around 1000 Å. Layer 303 is a singleorganic photoconductor, e.g., CuPc or PTCDA, or PTCBI deposited byorganic molecular beam deposition (OMBD) in a layer of thickness300-1000 Å, preferably about 500 Å. Second electrode 304 is transparent,e.g., ITO deposited by sputtering to an approximate thickness of1000-4000 Å, preferably less than 2000 Å and most preferably around 1000Å. An understanding from the prior art would indicate that such asymmetric unilayer device configuration would not generate a netphotocurrent. However, the necessary asymmetry is believed to arise atthe interface between organic photoconductor layer 303 and secondelectrode 304 from electronic surface states in organic photoconductorlayer 303 produced during the deposition of ITO second electrode 304.The damage region is schematically represented as region 303 a. Theexact mechanism by which the altered region at the interface producesasymmetric charge separating regions is not completely understood.Without limiting the present invention to a particular theory, theeffect of energetic electrode deposition, such as sputtering, may besufficient to alter the energy states at the interface so significantlythat the conventional picture of opposing mirror-image Schottky barriersis altered. Parthasarathy '031 discloses that the surface states, whichmay be defects, may effectively provide small energy “steps” that allowelectrons to more easily traverse what would otherwise be, for example,a Schottky barrier. One theory of unilayer device operation is thatdeposition of electrode 304 onto organic layer 303 creates a lowresistance contact, here a cathode. It is believed in this case that theresulting asymmetry would yield a net photocurrent.

Exemplary embodiments of an organic photosensitive optoelectronic devicewith two organic layers, or a bilayer device 400, are depicted in FIGS.4A and 4B. The substrate 401 provides support for first electrode 402which comprises ITO of approximate thickness 1000-4000 Å, preferablyless than 2000 Å and most preferably around 1000 Å. First organic layer403 is adjacent to first electrode 402 and comprises, e.g., PTCDA orPTCBI. Second organic layer 404 comprising, e.g., CuPc, is adjacent tofirst organic layer 403 to form the photovoltaic heterojunction.Finally, in 4A00 in FIG. 4A, second electrode 4A05 comprises ITO ofapproximate thickness of 1000-4000 Å, preferably less than 2000 Å andmost preferably around 1000 Å and is deposited onto second organic layer404. In the alternate embodiment 4B00 in FIG. 4B, the second electrodeis a metallic/non-metallic composite electrode comprised ofsemitransparent metallic layer 4B05 which is topped by an ITO layer4B06. Semitransparent metallic layer 4B05 may preferably consist of 10%Ag in 90% Mg, or other low work function metals or metal alloys. Layer4B05 is thin, preferably less than 100 Å thick. ITO layer 4B06 is about1000-4000 Å thick and is preferably less than 2000 Å and most preferablyapproximately 1000 Å thick. Both organic layers are approximately300-1000 Å thick and are preferably about 500 Å thick. The junctionformed at the first organic layer 403/second organic layer 404 interfaceis such that under electromagnetic radiation, excitons formed in thevicinity of the interface are dissociated to form free electron-holepairs. Photogenerated holes are collected at first electrode 402 andphotogenerated electrons are collected at second electrode 405.

Embodiment 4A00 incorporates the low resistance metal substitute cathodedisclosed in Parthasarathy '031. It should be observed that in contrastto the OLEDs of Parthasarathy '031 wherein the electrons move from thecathode into the adjacent organic semiconductor, organic photosensitiveoptoelectronic bilayer device 400 represents an embodiment of aphotosensitive optoelectronic device. Accordingly, the thicknesses ofthe photoconductive organic materials are adjusted to the requirementsof a photosensitive optoelectronic device, and during operationelectrons flow from second organic layer 404 into second electrode 4A05.Without being limited to this particular theory, it was typicallybelieved by one skilled in the art that metal substitute electrodes,e.g., ITO, would create a barrier to electron flow in both directions,i.e., into or out of the device, when such materials were used ascathodes. The low resistance operation of the ITO cathode in the presentinvention demonstrates that the surface states proposed in Parthasarathy'031 are effective in reducing the barrier to electron flow in eitherdirection at an ITO cathode made in accord with the present invention ofParthasarathy '031. Although first organic layer 403 and second organiclayer 404 may be interchanged in some cases and still obtain a workingdevice with only a reversal of polarity, these embodiments are lesspreferred due to the reduced efficiency of a device whenever the metalsubstitute electrode is used as the bottom cathode. The surface statesbelieved to effectively reduce the barrier at an ITO cathode are notthought to be produced when the organic thin film is deposited onto theITO layer. The term “low resistance metal substitute cathode” is, thus,used herein to refer to a metal substitute, e.g., ITO, prepared bysputter depositing a metal substitute, e.g., ITO, onto an organic layer,in particular, as distinct from an electrode prepared by depositing anorganic layer onto a metal substitute, e.g., ITO. It is believed hereinthat the barrier-reducing effect of surface states also can function topermit holes to traverse what would otherwise be blocking junctions atan interface between an organic semiconductor layer and an electrode orcharge transfer layer.

Embodiment 4B00 utilizes a transparent metallic/non-metallic compositecathode comprised of layers 4B05 and 4B06. Here again, as an embodimentparticularly suited for use as a solar cell, during operation electronsgenerated near the 403/404 interface move toward layer 4B05 and movethrough layer 4B06 for extraction. It is noted, in particular, that,whenever the ITO is sputter deposited onto a given thickness of Mg:Ag,the metallic/non-metallic composite cathode of Mg:Ag/ITO hassubstantially higher transparency than a noncomposite metallic layerhaving the same given thickness as the metallic part of the compositeelectrode. Further, the composite transparent composite layer may havebetter electronic properties than the lower resistance highlytransparent non-metallic cathode. Thus, while the transmission of themetallic/non-metallic composite electrode is typically not as high thehighly-transmissive low resistance metal substitute cathode, thetransmission is still sufficient, in combination with the superiorelectronic properties, for such metallic/non-metallic cathodes to beuseful in the photosensitive optoelectronic devices of the presentinvention.

In the bilayer cell, charge separation occurs in the vicinity of theorganic heterojunction whose built-in potential is determined by thesize of the HOMO-LUMO gap energy difference between the contactingmaterials, here first organic layer 403 and second organic layer 404.FIG. 5 depicts the relative HOMO-LUMO gaps of exemplary materials for anisotype heterojunction, e.g., CuPc/PTCDA. The proximity of the Fermienergy, EF, to the HOMO band shows that all of these materials haveholes as the majority carrier. The HOMO offset is equal to thedifference of ionization potentials (IP) of the two contacting materialsand the LUMO offset is equal to the HOMO offset plus the difference inHOMO-LUMO gap energies (EH-L) of the two materials. The polarity of thedevice is generally determined by the HOMO-LUMO energy differences. Forexample, in FIG. 5 since the LUMO of the PTCDA first organic layer 403is higher in energy than that in the CuPc second organic layer 404,electrons that are liberated from an exciton will move from the PTCDA403 into the CuPc 404 and contribute to the photocurrent. Absorption canoccur in either of the two organic films, thereby extending the width ofthe photoactive region where optical absorption can result in efficientcharge separation by the built-in field. Note that it is not criticalwhether a “p-n”-like junction or a simple isotype (i.e., p-P or n-N)heterojunction is employed, since it is only the diffusion potentialcreated at the heterojunction due to the HOMO-LUMO gap offsets that iseffective in carrier drift.

In the bilayer cells any rectifying effect of the organicmaterial/electrode junction is much less significant than that occurringat the bilayer heterojunction. Further, since the damage layer whichoccurs when, e.g., ITO, is deposited over an organic material issignificant in causing exciton ionization, this damage may be controlledusing the low power sputtering technique described in Parthasarathy'437. It is believed that controlling this damage region will insurethat exciton ionization and carrier separation primarily occurs at theorganic/organic bilayer heterojunction.

An exemplary embodiment of an organic photosensitive optoelectronic cellwith multiple organic layers, or a multilayer device 600, is shown inFIG. 6. Insulating or conducting substrate 601 supports the device.First electrode 602 comprises, e.g., ITO of approximate thickness1000-4000 Å, preferably less than 2000 Å and most preferably around 1000Å and is adjacent to first organic layer 603 comprising, e.g., PTCDA,PTCBI, or CuPc of approximate thickness 20-50 Å. A second organic layer604 comprises, e.g., 4,4′-bis[N-(1-napthyl)-N-phenyl-amino]biphenyl(α-NPD), approximately 200-500 Å in thickness, and is adjacent to firstorganic layer 603. A third organic layer 605, comprising, e.g., aluminumtris(8-hydroxyquinoline) (Alq₃), approximately 200-500 Å in thickness,is adjacent to second organic layer 604 to form a rectifyingheterojunction at the second organic layer 604/third organic layer 605interface. A fourth organic layer 606, comprising, e.g., CuPc, PTCBI, orPTCDA, of approximate thickness 20-50 Å is adjacent to third organiclayer 605. Finally, second transparent electrode 607 is adjacent to thefourth organic layer 606 and comprises, e.g., ITO of approximatethickness 1000-4000 Å, preferably less than 2000 Å and most preferablyaround 1000 Å. In this embodiment, an extra pair of organic materials,here second organic layer 604 and third organic layer 605, selected tohave appropriate relative mobilities and HOMO-LUMO offset for excitonionization and charge separation is placed within a “sandwich” of twoother organic materials, here first organic layer 602 and fourth organiclayer 606. In this instance, the “inner” pair of organic materials, 604and 605, provides the exciton ionization and charge separation and the“outer” pair, 603 and 606, serves both as charge transporting layers,i.e., transporting the separated carriers to the proper electrodes forsubstantially ohmic extraction, and as protective cap layers, i.e.,protecting the inner pair of organic layers from damage duringdeposition and use. The outer pair of organic materials may be from thegroup consisting of CuPc, PTCDA, and PTCBI, or any two of the three maybe used. That is, the same material or any combination thereof may beused for both contacts. Note, however, in embodiment 600, the interiorpair of layers, 604 and 605, are preferably deposited so that thecathode side is on top so as to incorporate a low resistance cathode.However, as with the exemplary embodiment of FIG. 4A, the order of thedeposition of the inner pair of organic materials is not criticalelectronically, though the order of the inner pair determines thepolarity of the photosensitive optoelectronic device. Since the outerpair of organic layers is relatively thin, their electronic propertiesare of much less significance here than in the bilayer exemplaryembodiment described herein above wherein the CuPc, PTCDA, and PTCBIalso performed photoconversion and exciton ionization in addition totransporting the separated carriers. Accordingly, an alternateembodiment of the present invention (not depicted) in a multilayerdevice would include the cathode on the bottom. The inner pair oforganic materials may each be an organic dye chosen to havephotosensitivity in a desired region of the spectrum. Since theAlq₃/α-NPD pair is photosensitive in the ultraviolet (UV) part of thespectrum, multilayer device 600 with this organic pair combination is aparticular exemplary embodiment of a UV photodetector. Further, the dyepair is preferably chosen to have a LUMO-HOMO gap offset as describedabove. In yet another embodiment (not shown) one or both of the outerpair of organic layers is replaced with a thin layer, approximately50-150A of Mg:Ag alloy which acts as a charge transfer, extraction, andprotective cap layer.

A further exemplary embodiment of the present invention is shown in FIG.7, which depicts a stacked single-organic-layer (unilayer)photosensitive optoelectronic arrangement 700 wherein the individualunilayer subcells are electrically connected in parallel. Insulating orconductive substrate 701 provides support for the stacked photosensitiveoptoelectronic device. The stack shown consists of three completeunilayer photosensitive optoelectronic subcells separated by transparentinsulating layers. Specifically, each subcell has a first transparentelectrode, e.g., ITO, 702 a, 702 b, and 702 c. On top of each first ITOelectrode there are organic layers 703 a, 703 b, and 703 c. Then on topof each organic layer is a second ITO electrode 704 a, 704 b, 704 c.Since the damage layer which produces the required device asymmetry isalways developed at the interface where ITO is deposited onto an organicmaterial, all of the devices will have the same electrical polarity withrespect to the direction of material deposition. That is, the subcellsare inherently deposited in an electrically series configuration. Such aseries configuration may be useful in low power applications asdiscussed above. Also, in accord with the present invention, externalconnections are possible to the intervening electrodes to allow for theavailability of multiple voltages from a single device comprisingstacked unilayer subcells. (This embodiment is not illustrated).However, a parallel electrical arrangement is preferable for high powerapplications in view of the series resistance and space charge effectsdiscussed above. This means that the subcells may not share common ITOelectrodes as is possible in the bilayer stacks discussed below.Therefore, in the embodiment shown in FIG. 7, transparent insulatinglayers 705 a and 705 b are provided to separate the adjacent subcells.This allows all the first ITO electrodes, 702 a, 702 b, and 702 c to beconnected to a common electrical path. Similarly, all the second ITOelectrodes 704 a, 704 b, and 704 c are connected to a common electricalpath. It can be appreciated that the stack could be continued to includea number of subcells. Stacked unilayer devices may be optimizedaccording to the criteria disclosed herein to, e.g., maximize totalvoltage, quantum efficiency or power. Stacking beyond a thickness inwhich substantially all incident light is absorbed will not furtherincrease conversion efficiency.

Two exemplary embodiments 8A00 and 8B00 are shown in FIGS. 8A and 8B.Embodiment 8A00 comprises several bilayer photosensitive optoelectronicdevices arranged so that their polarity alternates as a function ofposition in the stack. Insulating or conductive substrate 801 providessupport. Layers 802 a, 802 b, and 802 c are photosensitiveoptoelectronic subcell first transparent electrodes comprising, e.g.,ITO. They may also be some other transparent oxide or a metal. Layers803 a, 803 b, 803 c, and 803 d are photosensitive optoelectronic subcellfirst organic layers comprising, e.g., CuPc. Layers 804 a, 804 b, 804 c,and 804 d are photosensitive optoelectronic subcell second organiclayers comprising, e.g., PTCDA or PTCBI. Layers 805 a and 805 b arephotosensitive optoelectronic subcell second transparent electrodescomprising, e.g., ITO. Layer thickness ranges are the same as thosegiven for the individual bilayer subcell embodiment described withrespect to FIG. 4A. Since the subcells are deposited back-to-back, allelectrodes within the interior of the stack, i.e., 802 b and 805 a and805 b, are contacts to two different subcells and the entire stackarrangement is electrically connected in parallel. As used herein, theterm “back-to-back” is used to indicate that adjacent subcells haveantiparallel polarities. It can be appreciated here as well that thestack could be continued to include an arbitrary number of subcells toincrease the photocurrent. However, stacking beyond a thickness in whichsubstantially all incident light is absorbed will not further increaseconversion efficiency. Also, the choice of which organic layer todeposit first is not critical though it does determine the stackordering thereafter for a given stacked device.

In another exemplary embodiment 8B00 of a stacked device of severalback-to-back subcells, electrode layers 802 a, 802 b and 802 c areaugmented as now described. In order to take advantage where possible ofthe beneficial effect of a thin metallic layer combined with a layer ofITO, using what is herein referred to as a metallic/non-metalliccomposite electrode, thin semitransparent metallic layers, e.g., Mg:Ag,are added in several places. Metallic layer 8B06 is adjacent to ITOlayer 802 a. Metallic layers 8B07 and 8B08 are adjacent to ITO layer 802b. Metallic layer 8B09 is adjacent to ITO layer 802 c.

It is appreciated that in the electronically back-to-back configurationof stacked bilayer device 8A00, the subcell cathodes necessarily arealternately on top of the individual subcells as the stack is built frombottom up. This means that not all subcells will have what is referredto herein as the low resistance non-metallic cathode. In yet anotherembodiment (not shown), the subcells may be deposited with the organiclayers in non-alternating order as just described but with additionaltransparent electrode layers and insulating layers between theindividual subcells in a manner similar to that described above for theunilayer device. In this embodiment, the subcells may be connectedexternally in parallel and also have the cathode always on top to takeadvantage of the low resistance non-metallic cathode.

In other embodiments, a bilayer stacked device may be constructed inwhich the subcells are in series by not alternating the order of the twoorganic layers. In accord with the present invention such devices mayhave external connections to electrodes between subunits or may have thesubunits interconnected by charge transfer layers and isolated fromexternal circuits. FIG. 8C depicts a series connected device 8C00 withintervening electrodes. Layers 803 a, 803 b, 803 c, 803 d are arrangedto be on top of layers 804 a, 804 b, 804 c and 804 d in each of theirrespective subcells. This means that each subcell is arranged to have anETL, e.g., CuPc, on top of a HTL, e.g., PTCDA or PTCBI, so that eachsubcell has the cathode on top. This means that this embodiment isarranged to take advantage of the low resistance non-metallic cathode.Device 8C00 provides a plurality of negative voltages V_(L1 . . . N)where N is the number of subcells. N=4 is depicted in FIG. 8C. Note alsothat each available voltage V_(Li) is the sum of the voltages from alllower numbered cells. In another embodiment (not shown) which is avariant of device 8C00, the metallic/non-metallic composite cathode isutilized by inserting thin semitransparent metallic layers of, e.g., 10%Ag in 90% Mg, or other low work function metals or metal alloys. Thesemetallic layers of, approximately 100 Å or less in thickness, are placedbelow and adjacent to the metal substitute, e.g., ITO, layers 805 a, 802b, 805 b and 802 c to form a metallic/non-metallic composite cathode foreach subunit. In another embodiment (not shown) layers 804 a, 804 b, 804c and 804 d are arranged to be on top of layers 803 a, 803 b, 803 c, 803d so that the subunits have the anode on top and the voltages V_(Li)correspondingly have the opposite polarity. FIG. 8D depicts a seriesconnected device 8D00 similar to device 8C00 but with the subunitsinterconnected by charge transfer layers 8D11, 8D11 and 8D12 of, e.g.,ITO of approximate thickness 1000-4000 Å, preferably less than 2000 Åand most preferably around 1000 Å. There is yet another embodiment (notshown) in which the organic semiconducting layers are reversed so that adevice with an overall opposite electrical polarity is obtained. Anotherembodiment is a variation (not shown) of device 8D00 in which thinsemitransparent metallic layers of, e.g., 10% Ag in 90% Mg, or other lowwork function metals or metal alloys layers of approximate thickness 100Å or less are placed below and adjacent to layers 8D10, 8D11 and 8D12 toform metallic/non-metallic composite charge transfer layers and belowand adjacent to layer 802 c to form a metallic/non-metallic compositecathode.

In FIG. 9, an exemplary embodiment 900 of a stacked organicphotosensitive optoelectronic device comprised of a number of multilayerorganic photovoltaic subcells. Insulating or conductive substrate 901provides support. Photovoltaic subcell first transparent electrodes 902a, 902 b, and 902 c comprise, e.g., ITO. Photosensitive optoelectronicsubcell first organic layers 903 a, 903 b, 903 c, and 903 d comprise,e.g., CuPc or PTCDA or PTCBI. Photosensitive optoelectronic subcellsecond organic layers 904 a, 904 b, 904 c, and 904 d comprise, e.g.,α-NPD. Photosensitive optoelectronic subcell third organic layers 905 a,905 b, 905 c, and 905 d comprise, e.g., Alq₃. Photosensitiveoptoelectronic subcell fourth organic layers 906 a, 906 b, 906 c, and906 d comprise, e.g., CuPc or PTCDA or PTCBI. Photosensitiveoptoelectronic subcell second transparent electrodes 907 a and 907 bcomprise, e.g., ITO. Dimensions in each subcell of this exemplaryembodiment are the same ranges as in the embodiment of FIG. 6 but arevaried for optimization according to the criteria disclosed elsewhereherein. All first transparent electrodes 902 a, 902 b, and 902 c areelectrically connected in parallel and all second transparent electrodesare electrically connected in parallel to provide increasedphotocurrent. Here as well, the stack could be continued to include anarbitrary number of subcells. However, stacking beyond a thickness inwhich substantially all incident light is absorbed will not furtherincrease conversion efficiency. Just as the order of the inner pairs ofthe subcell was not critical in the exemplary embodiment of FIG. 6, itis not critical here either except that the pair order must alternatebetween subcells to allow the interior transparent electrodes to act ascommon contacts for adjacent cells. Therefore, the order of the firstinner pair of organic layers will determine the order for the remainderof the stack in this parallel interconnected embodiment.

A further embodiment of the present invention (not shown) is a variationof embodiment 900 and has the subcells separated by transparentinsulating layers, such as SiO₂ of approximate thickness 500-4000 Å,preferably less than 3000 Å and most preferably around 2000 Å, withadditional transparent electrode layers as required in analogy to theunilayer and bilayer stacked devices described above. If the Alq₃ andα-NPD layers are arranged with the Alq₃ on top then the subcells are alloriented with the cathode on top to utilize the low resistance cathodesuch as disclosed in Parthasarathy '031. With the subcells thuselectrically isolated in the stack they may be connected externally inaccord with the present invention in either a parallel or series manneras described with the bilayer subcells.

In other variations of embodiment 900, the present invention has astacked configuration wherein the subunits are stacked in series, i.e.,with aligned polarity but not separated by insulating layers. In oneembodiment (not shown) the multilayer cells are interconnected byelectrode layers such that a plurality of voltages are available from asingle device similar to embodiment 8C00. In another variation (notshown) the adjacent multilayer subcells are interconnected by chargetransfer layers in a manner similar to embodiment 8D00. In all of thevariations (not shown) of embodiment 900 described herein, themetallic/non-metallic composite cathode such as disclosed in Forrest'436 may be used in place of the highly transparent, low resistancenon-metallic cathode. In this case the fourth organic layer of eachsubcell, 906 a, 906 b, 906 c, and/or 906 d, may be eliminated, ifdesired.

In yet another embodiment (not shown) one or both of the outer pair oforganic layers in the subcells is replaced with a thin layer,approximately 50-150 Å, of Mg:Ag alloy which acts as a charge transfer,extraction, and protective cap layer.

It should be noted that in all of the stacked devices so far discussed,e.g., unilayer, bilayer, and multilayer, the thicknesses of all thesubunits in any given device may be uniform, i.e., substantially thesame from one subcell or subunit to the next. In particular, thethicknesses of the photoconductive organic layers which performphotoconversion are approximately the same in each subunit. However,since the photosensitive layers absorb electromagnetic radiation, theintensity of radiation admitted on one side, e.g., the top, of a stackeddevice decreases as the radiation traverses deeper into the device. Thevoltage generated by each cell or subcell is generally a function ofcertain intrinsic properties, e.g., the energy level distributions inthe respective materials forming the heterojunction and in particularthe HOMO-LUMO gap as previously discussed herein. However, the currentgenerated in a cell or subcell depends on the amount of electromagneticradiation which is admitted to the particular cell or subcell. In astacked device composed of multiple subcells of uniform thickness andexposed to ambient electromagnetic radiation from only one direction,the current generated by subcells farther from the admission surface, orface, of the stacked device will decrease in cells progressively fartherfrom the face. In stacked devices which are configured to beelectrically connected in parallel, e.g., embodiments 8A00 and 900, thiswould not necessarily be a major drawback since the currents arecombined externally to supply a current equal to the sum of that fromall subcells with the voltage fixed by the intrinsic uniform voltage ofthe subcells. In such parallel devices the total power output of eachsubcell may be extracted without any inefficiency due to limitation by aparticular subcell. In situations where it is desirable to obtainmultiple outputs having the same current capacity, devices may beconstructed as described below.

On the other hand, the different current limitation in each subcell maybe an important factor in the operation of series connected stackeddevices. In a stacked device which is connected in series such asembodiment 8D00, fundamental current continuity considerations constrainthe device's current output so that it is limited to the current whichgoes through the subcell generating the least current regardless of therelative position of a subcell in the stack. To address this problem,other embodiments of the present invention utilize subcells havingphotoconductive organic layers with thicknesses which varysystematically between subcells. For example, in a variation ofembodiment 8D00, the photoconductive organic layers vary in thicknesssuch that each subcell has exponentially thicker photoconductive organiclayers if measured starting at the top of the device. This lastvariation of embodiment 8D00 would be most appropriate where the sourceof the electromagnetic radiation is limited to one side of the device,here the top, i.e., farthest from the substrate. In another variation,the photoconductive organic layers may be made exponentially thickerstarting from the bottom subcell. This embodiment would be appropriatein an environment where the source of electromagnetic radiation is onthe substrate side of the device when the substrate is transparent. Foruniformly illuminated environments where approximately equal intensityelectromagnetic radiation light is incident on the two faces of adevice, yet another variation of embodiment 8D00 may be efficient. Ifthe electromagnetic radiation has sufficient intensity that asignificant amount of the radiation incident upon each face is able totraverse the device, then an embodiment variation of 8D00 wherein thephotoconductive organic layers in the subcells in the center are thickerthan corresponding layers in subcells nearer the two faces will provideuniform current levels from each cell.

The situation with series stacked devices which generate multipleindependently accessible voltages, e.g., 8C00, is somewhat morecomplicated. In such a device, the currents through different subcellsin the device may be different since the multiple external connectionsto the various internal electrodes allow multiple paths for current toflow so the effect of low current production by any particular subcelldepends on the load dynamics.

In other embodiments of the present invention multiple layers of organicphotosensitive materials are used to enhance device sensitivity. In oneembodiment, organic photosensitive optoelectronic devices in accord withthe present invention incorporate multilayer photoconductor layers toenhance exciton dissociation. Specifically it is believed that thepresence of numerous well defined interfaces between materials providemore extensive exciton dissociation regions. The composite layers mayhave thicknesses commensurate with the device guidelines described aboveor thinner layers as described next.

In yet a further embodiment, organic photosensitive optoelectronicdevices in accord with the present invention incorporate multiplequantum wells (MQWs). MQWs are believed to alter the distribution ofexciton energy levels resulting in, inter alia, possible changes in thespectral sensitivity of MQW-containing devices as compared to deviceshaving the same materials and thicker photosensitive layers that do notexhibit quantum size effects. Also, it is believed that the changes inexciton energy levels may serve to enhance exciton dissociation. It isbelieved that these embodiments having multiple photosensitive layersincluding MQW structures provide enhanced photosensitivity but may notbe appropriate for continuous use since they can quickly saturate due tocharge which becomes trapped in the layers. Therefore, in an exemplaryuse organic photosensitive optoelectronic devices in accord with thepresent invention and incorporating MQWs would be used as intermittentphotodetectors with, for example, a liquid crystal shutter mechanismwhich would periodically block light for short intervals from thedetector so as to allow the trapped charge to dissipate. Such structuresmay be operated with a purely passive load as depicted in FIGS. 10A and10B. However, these multilayer structures are highly efficient whenoperated with an applied bias. Under such conditions such a device canproduce a large transient current when initially exposed to light.

Arbour et al. describes bilayer and multilayer assemblies of vanadylphathalocyanine (VOPC) and PTCDA using semitransparent Au or SnO₂ thinfilms as one contact, and transparent electrolytes, saturated in a redoxspecies for the opposing electrical contact. It is appreciated thatArbour did not utilize solid state electrode materials suitable forpractical devices for both electrical contacts. Also, it is appreciatedthat Arbour's multilayer devices did not have intervening electrodes orcharge transfer layers as disclosed herein above. Forrest Chem. Rev.1997 further describes the effect of multiple layers of organicphotosensitive materials but does not disclose the use of such layers indevices having two transparent electrodes.

Embodiments with numerous well defined interfaces and/or MQWs are nowdescribed with reference to FIG. 10A and FIG. 10B. FIG. 10Aschematically depicts a device 10A00 with two electrodes 10A02 and10A05, which are, e.g., ITO of approximate thickness 1000-4000 Å,preferably less than 2000 Å and most preferably around 1000 Å. For thisembodiment, the bottom ITO electrode preferably functions as the anodeand the top electrode functions as the cathode. For example, the top ITOelectrode may be sputter deposited onto the underlying organic layer soas to form a low resistance non-metallic cathode or, alternatively, thetop electrode 10A05 may be comprised of a thin Mg:Ag layer onto whichthe ITO layer is sputter deposited so as to form a metallic/non-metalliccomposite cathode. The layers 10A03 (a, b, c, d, e, f, g, h, i, and j)and 10A04 (a, b, c, d, e, f, g, h, i, and j) are pairs of organicphotoconductive materials which form a plurality of interfaces betweeneach respective 10A03 and 10A04 pair. The layers may be organicmolecular crystal semiconducting materials as described elsewhere hereinor polymer semiconductors as also described herein. For example, in oneembodiment the pairs may be, e.g., PTCDA/CuPc, PTCBI/CuPc, orPTCDA/VOPc. Note also that the layer pairs need not necessarily be ofdifferent majority carrier type but may be isotype. The importantfeature of the layer pairs is that they produce numerous interfaces topromote and in some cases alter exciton dissociation dynamics. Layers ofthe exemplary materials just mentioned may be in the range ofapproximately 20-1000 Å with 500 Å being the preferred upper end of therange. Devices designed specifically to utilize MQW effects will havemuch thinner layers with thicknesses determined by the approximatespatial dimensions of the primary excitonic modes. These considerationsare discussed in Forrest Chem. Rev. 1997 which is incorporated herein byreference. For the materials mentioned above, PTCDA/CuPc, PTCBI/CuPc,and PTCDA/VOPc, MQW layers should be in the 20-50 Å range. Embodiment10B00 in FIG. 10B is a variation of 10A00 having an additional thinmetallic layer 10B06 of, e.g., 50-150 Å of Mg:Ag alloy which acts as acharge transfer, extraction, and protective cap layer and forms ametallic/non-metallic composite electrode with the metal substitute ITOlayer 10A05.

It is appreciated that in these devices the photoconductive layermaterials may be deposited with either member of the pair on top as longas the layers are alternated throughout the device. Further, it isappreciated that the number of pairs depicted is illustrative only andmay be more or fewer depending upon the thicknesses of thephotoconductive layers. The general constraint again is that the totalthickness of all the layers should not generally be greater than thepenetration length of the electromagnetic radiation for which the deviceis to be utilized.

In yet a further exemplary embodiment, a reflective substrate of, e.g.,metal, or a non-reflective substrate coated with a reflective layer of,e.g., metal such as Al or Au, may be used to support organicphotosensitive optoelectronic device structures in accordance with thepresent invention. FIG. 11A depicts an example of an embodiment with areflective surface based on a single bilayer cell. 11A01 is a substrate,which may be transparent, e.g., glass, or non-transparent, e.g. a metal.Thin metal layer 11A02 is deposited on the substrate. Layer 11A03 is anoptional metal substitute electrode layer, for example, ITO. If layer11A03 is not used then layer 11A02 functions as an electrode and is of asuitable material as described elsewhere herein. First organic layer11A04 is an organic semiconductor, for example, CuPc. Second organiclayer 11A05 is a second organic semiconductor, for example, PTCDA orPTCBI. The transparent electrode 11A06 that is placed on top ispreferably a low resistance non-metallic cathode or ametallic/non-metallic composite cathode. FIG. 11B shows a stackedbilayer device embodiment with a reflective bottom layer with thesubcells connected in parallel. 11B01 is a substrate, which may betransparent, e.g., glass, or non-transparent, e.g., a metal. If a glasssubstrate is used, thin metal layer 11B02 may be deposited on thesubstrate. Since a metal substrate or metal layer is present to providethe reflective surface, the metal substrate or metal layer alsopreferably functions as the bottom electrode of the device. However, ifdesired, an optional transparent electrode layer 11B03 a comprised ofITO may be present. Layers 11B03 (b and c) are subcell first electrodelayers and may be a metal substitute such as ITO, a transparent metal,or comprise ITO over a thin metallic layer, such as Mg—Ag. Layers 11B06(a and b) are subcell second electrode layers and may be a metalsubstitute such as ITO, a transparent metal, or comprise ITO over a thinmetallic layer, such as Mg—Ag. Organic layers 11B04 (a, b, c and d) area first organic semiconductor, e.g., CuPc. Organic layers 11B05 (a, b, cand d) are a second organic semiconductor, for example, PTCDA or PTCBI.In another embodiment (not depicted), a transparent substrate is used tosupport any organic photosensitive optoelectronic device in accord withthe present invention. In this embodiment the top subcell of the stackhas a top reflective layer, typically and all intervening subcells havetransparent electrodes and light is received through the substrate. Thetop reflective layer can be an electrode or a metallic layer depositedon a transparent non-metallic electrode. In all of these embodiments,i.e., with either a top or a bottom reflecting surface, the reflectivesurface redirects any unabsorbed light back through the absorbing layersto allow further opportunity for photoconversion and enhancement of thequantum efficiency. Inclusion of a reflective layer reduces the numberof subcells required in a stack to obtain optimal photoconversionefficiency.

In fabricating practical organic photosensitive optoelectronic devices,in addition to determining the optimal number of and thickness of layersto have in a stacked photosensitive optoelectronic device as discussedabove, the area and arrangement of individual cells may be chosen tooptimize efficiency and cost. Since the transparent electrodes that areto be used in these devices do have some residual absorption, it ispreferable to keep such electrodes thin to minimize this absorption. Inaddition, keeping the electrode layers as thin as practicable minimizesfabrication time. On the other hand, as all electrode materials are madethinner, their sheet resistance increases. Therefore, it is preferableto minimize the distance which charge carriers must travel in theelectrode after collection. A configuration which maximizesphotosensitive optoelectronic receptive area and accomplishes this goalis one in which the devices are laid out in long strips on the substratewith electrical connection made to the devices electrodes along thelonger sides. U.S. patent application Ser. No. 08/976,666 to Forrest etal. (now U.S. Pat. No. 5,953,587; hereinafter Forrest '587), which isincorporated herein by reference in its entirety, describes techniquesfor fabrication of practical organic thin film devices.

Parthasarathy '031 disclosed that small molecular organic materials withrelatively large planar molecules and a highly conjugated structure,such as CuPc and PTCDA, are believed to be able to withstand energeticelectrode deposition because of the highly delocalized nature of theirbonding which spreads the energy of impact over more than one bond. Whenan energetic metal or oxygen atom is incident on one of these moleculesat a surface during sputtering, the energy of impact is thought to beefficiently distributed over the numerous bonds in the molecularπ-electron systems. In contrast, no comparably large π-electron systemsexist in, e.g., Alq₃ or α-NPD. For such molecules, the impact energy ismore localized among only a few atomic sites, thereby increasing theprobability for breaking a molecular bond. The planar or nearly planarstacking arrangements of crystalline molecular systems such as CuPc andPTCDA may also assist in the dissipation of energy among severalneighboring molecules in the lattice. Accordingly, it is believed thatthe low resistance non-metallic cathode such as disclosed inParthasarathy '031 can also be embodied in other similar types of smallmolecular crystals. For example, CuPc may be replaced by other metalphthalocyanines, naphthalocyanines and porphyrins. PTCDA may be replacedfor example by other polyacenes. Some other representativephotosensitive optoelectronic small molecule organic heterostructurecombinations contemplated as suitable for use in accord with the presentinvention are shown in Table 1. TABLE 1 Ag/PTCBI/CuPc/ITOIn/PTCDA/CuPc/ITO Au/H₂Pc/DM-PTCDI/ITO Au/H₂Pc/DM-PTCDI/PTCBI/ITOAu/H₂Pc/PTCBI/ITO Al/ZnPc/Au Au/ZnPc/DM-PTCDI/ITO In/PPEI/PVP(TPD)/ITOAu/CuPc/DM-PTCDI/ITO Au/ZnPc/DM-PTCDI/ITO Au/H₂Pc/PTCBI/ITOAu/TPP/DM-PTCDI/ITO Au/TBP/DM-PTCDI/ITO Au/H₂Hc/DM-PTCDI/ITOAu/H₂Pc/DM-PTCDI/ITO (Au/H₂Pc/DM-PTCDI)₂/ITO Au/(H₂Pc/DM-PTCDI)₂/ITOAl/C₆₀/TiOPc/ITO Al/C₆₀/VOPc/ITO Al/C₆₀/PPV/ITO Al/merocyanine/AgPPEO: 3,4,9,10-perylenetetracarboxyl N,N′-bis(phenylethylimide).PVP(TPD): 55 wt % N,N′-diphenyl N,N′-ditolylbenzidine inpoly(vinylpyridine) spin coated onto ITO surface prior to PPEIdeposition.TPP: 5,10,15,20-21H,31H-tetraphenylporphyrin.TBP: tetrabenzoporphyrin(29H,31H-tetrabenzo[2,3-b:2′3′-g:2″,3″-l:2′″,3′″-q]porphyrazine).H₂Nc:naphthalocyanine(29H,31H-tetranaphtho[2,3-b:2′3′-g:2″,3″l:2′″,3′″-q]porphyrazine).H₂Pc: phthalocyanine.PPV: poly(phenylene vinylene).ZnPc: zinc phthalocyanine.DM-PTCDI: N,N′-dimethyl-3,4,9,10-perylenetetracarboxylic diimide.TiOPc: titanium oxide phthalocyanineC₆₀: buckminsterfullerene.VOPc: vanadyl phthalocyanine.

Additionally, organic polymers may be used in accord with the presentinvention. Yu, G., Gao, J., Yang, C., and Heeger, A., “PhotovoltaicCells Made with Organic Composites”, Proceedings of the 1st NRELConference on Future Generation Photovoltaic Technologies, AmericanInstitute of Physics, March 1997, incorporated herein by reference inits entirety, discloses polymer-based solar cells using purepoly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV)and MEH-PPV blended with buckminsterfullerene (C₆₀). These materials arebelieved to be suitable as photoconducting materials in accord with thepresent invention. Also, “The Handbook of Conducting Polymers”,Skotheim, T., Ed., Marcel Dekker, 1986, especially Chapter 17: PolymericSemiconductor Contacts and Photovoltaic Applications”, Kanicki, J.,discloses a number of photoconductive polymers which are known in theart and which are believed to be suitable for use in devices made inaccord with the present invention. These include poly-2-vinylpyridine(PVP), polyphenylacetylene (PPA), polyphenylene (PPP), polyphenylenesulfide (PPS), polypyrrole (PPY), polyacrylonitrile (PAN),polyheptadiyne (PHD), polymethylacetylene (PMA), polyphenylene vinylene(PPPV), polyphenylene oxide (PPPO). These materials may be incorporatedin any of the several stacked photocell embodiments disclosed above.Also, it is believed that polymers that have a highly connectedstructure can form the low resistance non-metallic cathode by receivingan energetically deposited metal substitute cathode, such as sputteredITO, when such an electrode/semiconductor interface would otherwise forma substantial barrier to carrier flow.

Also, it will be appreciated that the novel concept of electricallyconnecting organic photosensitive optoelectronic devices in parallel toovercome the inefficiencies associated with their high series resistanceand related space charge effects is equally applicable to other organicphotosensitive optoelectronic devices comprising other subcellcompositions. Similarly the novel concept of connecting stacked organicphotosensitive optoelectronic devices in series with externalconnections to intervening electrodes to provide a multivoltage powersupply is applicable to other organic photosensitive optoelectronicdevices. Also, use of the low resistance non-metallic cathode disclosedin Parthasarathy '031 in place of a traditional semi-transparentmetallic cathode, such as disclosed in prior art organic photosensitiveoptoelectronic material configurations, is intended to be within thescope of the present invention.

General techniques for fabrication of devices in accord with the presentinvention are well-known in the art. The preferred method of depositingthe organic layers is by thermal evaporation; the preferred method ofdepositing metal layers is by thermal or electron-beam evaporation; thepreferred method of depositing dielectrics is by plasma-enhancedchemical vaporization, RF or RF/DC sputtering, or electron-beamevaporation. The fabrication of the organic photosensitiveoptoelectronic devices may be accomplished, for example, with either ashadow masking technique or shadow masking combined with a dry etchingprocess for preforming the substrate. Shadow masking and dry etching areknown in the art. Advanced photolithographic deposition techniques forproduction of the various embodiments of the present invention have beenpreviously disclosed in relation to the fabrication of organic thinfilms in Forrest '587. For example, one of ordinary skill in the art canconstruct the device embodiment as depicted in FIG. 12 using thetechniques described in Forrest '587 and incorporated herein with thefurther benefit of this disclosure.

FIG. 12 depicts an implementation of a stacked photosensitiveoptoelectronic device having three subcells with subcells connectedelectronically in parallel. Embodiment 1200 is depicted within the“shadow box” after the step of encapsulation as described in Forrest'587. 1201 is a continuous region of a transparent conductive material,e.g., ITO which forms a bottom electrode and one electrode in the middleof the stack. Layer 1202 is a plurality of photoconductive organiclayers (depicted as one here for convenience) which form a rectifyingjunction in accord with the present invention. 1203 is anothercontinuous region of transparent conductive material, e.g., ITO whichforms a top electrode and one electrode in the middle of the stackeddevice. Layers 1204 and 1205 are similarly pluralities ofphotoconductive organic layers. It is appreciated that in thisembodiment there are three subcells which are deposited so as to havealternating polarity, i.e., the top and bottom subcells have the samepolarity and the middle subcell has the opposite polarity. It isappreciated in embodiment 1200 that the photoconductive organic layers1202, 1204 and 1205 are intended to be continuously connected. Theorganic materials contemplated for use in this particular embodiment aresufficiently poor conductors that they can effectively insulate thetransparent conductive regions 1201 and 1203 from each other as depictedand still perform the photoconductive functions described elsewhereherein.

Protective layer 1206 should be made of a nonconductive material. Forexample, the protective layer could be spin coated polymer, such asTEFLON™, or sputter deposited SiO₂ or Si₃N₄. Protective layer 1206should also be transparent when electromagnetic radiation is to beadmitted through the top of the device. In another optional embodiment(not illustrated), protective layer 1206 may be omitted but the topelectrode layers must be made thick as to be impervious to ambientatmosphere to protect the organic materials comprising the device fromperformance degrading chemical reactions.

Using the “shadow box” technique a stacked bilayer organicphotosensitive optoelectronic device similar to FIG. 12 can be made inthe following steps:

-   1) Depositing a transparent 5-10 μm dielectric layer onto a    substrate with prepatterned contacts. The dielectric layer can be    SiO₂, for example.-   2) Depositing a photoresist layer.-   3) Exposing the photoresist layer to light in a pattern for device    bottom layers.-   4) Removing unexposed photoresist areas to leave a pattern of    photoresist over the dielectric layer.-   5) Remove the dielectric layer by, e.g., chloride reactive-ion    etching to leave strips of dielectric covered with photoresist and    wet etching to create the “undercut”.-   6) Angle deposit a first ITO layer.-   7) Angle deposit a CuPc layer.-   8) Angle deposit a PTCDA layer.-   9) Angle deposit a second ITO layer.-   10) Angle deposit a PTCDA layer.-   11) Angle deposit a CuPc layer.-   12) Angle deposit a second ITO layer.-   13) Repeat steps 7-12 to build up stack. The stacking can be stopped    on repetition of either step 9 or 12.

Using a conventional shadow masking technique a stacked bilayer organicphotosensitive optoelectronic device similar to FIG. 12 can be made inthe following steps:

-   1) Deposit a first ITO layer by shadow masking onto a substrate with    prepatterned contacts.-   2) Deposit a CuPc layer by shadow masking.-   3) Deposit a PTCDA layer by shadow masking.-   4) Deposit a second ITO layer by shadow masking.-   5) Deposit a PTCDA layer by shadow masking.-   6) Deposit a CuPc layer by shadow masking.-   7) Deposit a second ITO layer by shadow masking.-   8) Repeat steps 2-7 to build up stack. The stacking can be stopped    on repetition of either step 4 or 7.

Forrest '587 also describes techniques for fabricating arrays ofoptoelectronic devices which are also applicable to the presentinvention. With these techniques described in Forrest '587 and thisdisclosure one of ordinary skill can construct an array (not depicted)of organic photosensitive optoelectronic devices which is particularlysuitable as a multipixel photodetector. Such an array is capable ofdetecting electromagnetic radiation with spatial and intensityresolution.

It will also be appreciated that the substrate may be any smoothmaterial such as transparent materials glass, quartz, sapphire orplastic, or opaque materials such as silicon or metal and includingflexible material such as plastic films, e.g., polystyrene, or metalfoils, e.g., aluminum foil. Any roughness present on the surface of thesubstrate can be smoothed by depositing an optional thin layer ofpolymer on top of the substrate, or by applying a similar smoothingtechnique. Though the exemplary embodiments generally describe organicphotosensitive optoelectronic devices deposited onto insulatingsubstrates, it will be appreciated that if a conductive metal is used asthe substrate it may be used as a first electrode in lieu of an appliedfirst electrode. Alternatively, an insulating layer of, e.g., aninsulating oxide, may be placed over the metal before the device isdeposited as described in the above exemplary fabrication method.

The organic photosensitive devices of the present invention may beincorporated into, inter alia: light powered radios, televisions,computers, calculators, telephones and other wireless communicationsdevices, watches, emergency location devices, electric vehicles, powergeneration systems and devices, and emergency power supplies; monitoringand detection equipment for power and/or sensing, inspection devices,radiation detectors, imaging devices; and optical coupling devices forelectrical isolation or use in fiber optic communications.

Those skilled in the art will recognize that the devices described abovemay be used as solar cells or photodetectors. That is, when such adevice is exposed to electromagnetic radiation while a bias voltage isapplied, the device is in photodetector mode and a current is producedthat correlates with the intensity of the incident radiation. In solarcell operation, no bias voltage is applied and power is directed throughan external load. Accordingly, the present invention and claims areintended to cover these devices regardless of which of these modes isused when operating the device.

Thus, there has been described and illustrated herein an organicphotosensitive optoelectronic device and method for producing the same.Those skilled in the art, however, will recognize that manymodifications and variations besides those specifically mentioned may bemade in the apparatus and techniques described herein without departingsubstantially from the concept of the present invention. Accordingly, itshould be clearly understood that the form of the present invention asdescribed herein is exemplary only and is not intended as a limitationon the scope of the present invention.

1-53. (canceled)
 54. A system comprising: a first electrode and a secondelectrode; a plurality of organic photovoltaic cells arranged in a stackdisposed between the first electrode and the second electrode, each cellcomprising a rectifying junction at an interface of organicsemiconductor materials, a first cell and a second cell of the pluralityof organic photovoltaic cells having different absorptioncharacteristics; metal or metal substitute disposed in the stack betweeneach of the cells; and a resistive load electrically connected acrossthe first electrode and the second electrode, photocurrent from theplurality of organic photovoltaic cells to energize the resistive load.55. The system of claim 54, wherein the resistive load comprises a powerconsuming device.
 56. The system of claim 54, wherein the resistive loadcomprises a power storing device.
 57. The system of claim 54, whereinthe first cell and the second cell have different absorption maxima. 58.The system of claim 54, wherein the plurality of organic photovoltaiccells are electrically connected in series.
 59. The system of claim 58,wherein the first cell and the second cell also have differentthicknesses.
 60. The system of claim 59, further comprising a reflectivesurface positioned to reflect light that passes through the stack backinto the stack, the first cell being closer to the reflective surfacethan the second cell, wherein the first cell is thicker than the secondcell.
 61. The system of claim 59, wherein the first electrode and thesecond electrode are transparent and the plurality of organicphotovoltaic cells includes a third cell, the second cell beingpositioned in the stack between the first cell and the third cell, andwherein the second cell is thicker than the first cell and the secondcell is thicker than the third cell.
 62. The system of claim 54, whereinthe plurality of organic photovoltaic cells are electrically connectedin parallel.
 63. The system of claim 54, wherein the plurality oforganic photovoltaic cells are electrically connected in a combinationof parallel and series.
 64. The system of claim 54, wherein the metal ormetal substitute disposed in the stack between at least one pair of thecells forms an intermediate electrode.
 65. The system of claim 64,wherein the intermediate electrode is a compound electrode includingboth metal and metal substitute.
 66. The system of claim 54, wherein themetal or metal substitute disposed in the stack between at least onepair of the cells forms a charge transfer layer.
 67. The system of claim54, wherein the metal or metal substitute is an elementally pure metalor a metal alloy.
 68. The system of claim 54, wherein the metal or metalsubstitute is a transparent conducting oxide.
 69. The system of claim54, wherein the metal or metal substitute is a transparent conductivepolymer.
 70. The system of claim 54, wherein the first electrode and thesecond electrode are transparent.
 71. The system of claim 54, wherein atleast one of the first electrode and the second electrode is a compoundelectrode including both metal and metal substitute.